Rebreather FAQ

Rebreathers: The Basics

First, a Few Questions:

Are you cold after a couple of dives even though you are
wearing an appropriate exposure suit?

Are you seeing less marine life on your dives than you
should?

Do you have to surface because your gas supply is depleted
before you're ready to leave?

Does your back hurt from carrying doubles and stage
bottles?

Would you like to be able to dive with Helium-based gasses,
without spending hundreds of dollars per dive?

Would you like a way to accelerate decompression, yet carry
fewer stage/deco bottles?

If you answered "yes" to any of the above questions, you might
be ready to try diving with a rebreather. In addition to avoiding
these issues, you would get out of the water feeling great, and
also be warmer and hydrated. You probably already enjoy the
benefits of Nitrox diving. Rebreather diving is even better. The
dive itself becomes almost silent. Couple that with minimized
buoyancy changes from breathing to achieve a gliding effect.
Diving a rebreather feels more like flying. You merge with the
dive site and your environment, rather than existing as a noisy
distant observer. You're really swimming with the fish,
rather than just watching them swim away. And all while breathing
comfortably and normally.

More and more in the dive magazines, there is a growing buzz
about rebreathers. These devices are not new. They have, in fact,
been around much longer than the now traditional open circuit
SCUBA. The earliest rebreathers were made from pigskin bladders.
Before anyone gets any ideas and takes that old football and
garden hose into the garage, realize that the technology has
advanced a lot since 900 BC.

It is modern technology which is now bringing rebreathers into the
reach of more than the just the military and the rich. Technology is
improving and lowering costs in manufacturing. Digital controls
and microprocessors have made electronics smaller and more
survivable in a hostile marine environment. In short,
technology has finally caught up with the concept.
Technology is making rebreathers more affordable, safe, and
practical.

What Is A Rebreather?

There are several categories of rebreathers, but let's start
with the basics. A rebreather is a device which re-circulates the
diver's breathing gas. A typical person consumes less than 10% of
the Oxygen (O2) in the breathing gas in every breath.
This means that the other 90+% is wasted if it is just exhaled,
and not "rebreathed". There are three primary categories of
rebreathers. They vary from Oxygen rebreathers to Semi-Closed
Circuit (SCR) to state of the art Fully-Closed Circuit
Rebreathers (CCR). These systems vary in the manner in which gas
is added or replenished. All rebreathers must scrub out the
CO2 from the breathing gas; otherwise the diver will
become hypercapnic (which can cause blackout and other
problems)

The Basic Rebreather Consists of:

Breathing loop:

Mouthpiece. (Dive Surface Valve or DSV).

Counter-Lung(s). To contain the diver's exhaled gas for
processing and rebreathing.

Scrubber. To remove carbon dioxide (CO2)
from the breathing gas.

Check Valves. To keep gas flow directional.

Breathing Hoses. To connect the DSV, Counter-Lung(s),
and Scrubber.

Gas Addition/Replenishment System.

Compressed gas cylinder(s).

Regulators. To reduce the high pressure gas in the
cylinders to ambient pressure.

Oxygen measuring device.

Gas addition controls.

Oxygen Rebreathers:

In this design, only pure O2 is injected into the
breathing loop. This is done via a demand valve and manual
override. These are Fully Closed Circuit systems which are
extremely depth limited because the diver is breathing pure
O2. The typical use is military. These are the only
class of rebreathers which are totally "bubbleless". There is
never a decompression obligation with these units, as there is no
inert gas in the system to be absorbed by the diver's
tissues.

Semi-Closed Rebreathers (SCR):

In this design, Nitrox (O2 enriched air) is
injected into the breathing loop via a mass flow orifice valve,
or a system which keys the injection to the divers breathing
rate. Because gas is continually injected into the loop, it must
also be expelled from the loop (hence the term semi-closed).
These systems offer gas use economy over open circuit SCUBA, but
there is no decompression advantage over nitrox open circuit
diving.

Fully Closed Circuit Rebreathers (CCR):

In this design, there are two gases; pure O2 and a
diluent gas (air, tri-mix or heliox, etc). Diluent is added
either automatically or manually to the loop to make up the bulk
of the loop volume during descent. In a CCR system, only
O2 is consumed or burned by the diver. That
consumption rate is based on the diver's metabolic rate. It is
not affected by the depth of the dive, but by the diver's
workload. This means that the diver will consume the same amount
of O2 at 300 ft as at 30 ft or 3ft, provided he is
performing the same task.

CCR's require the gas in the breathing loop to be monitored.
This is done via oxygen sensors (typically 2 or 3). O2
is injected into the loop to maintain a set partial pressure of
O2. There are two injection strategies within this
topic. Electronically Controlled (automatic) and Manually
Controlled. Some CCR's are designed to allow both strategies.

CCR's offer the maximum decompression advantage over open
circuit SCUBA. CCR's are truly the best choice if your diving
ranges from advanced recreational to heavy technical.

Q & A

Q: What Are Some Basic Differences Between Rebreathers
and Open Circuit SCUBA?

A: Open circuit SCUBA has seen a great deal of development
over the years, and now even low-end regulators breathe better
than most of the high-end regulators of a decade ago. Of course,
further improvements can be made.

When using open circuit SCUBA, regardless of model or
manufacture, after each breath you exhale into the surrounding
water. That exhaled gas is gone. The deeper the diver goes, the
greater the volume of gas that is expelled on every breath. The bottom time
with open circuit SCUBA is typically limited by how deep the
diver is going and how much gas can be carried. By comparison, in
a CCR where only O2 is consumed, bottom time is
limited by scrubber duration (typically 3 to 5 hours), decompression
obligation (optimized), and oxygen toxicity (CNS and OTU's).

SCUBA cylinders are cooled due to adiabatic expansion as the
gas leaves the tank. With open circuit SCUBA, the breathing gas
coming from the tank will almost always be at a lower temperature
than the surrounding water by the time the diver inhales it. The
diver's body warms the incoming gas only to have it expelled on
the next breath. This removes heat directly from the diver's
core. The open circuit diver is also breathing very dry
gas. This is hard on the body because the lungs require moisture
for efficient gas exchange. The body must hydrate the incoming
gas, primarily in the lungs. This moisture is then expelled into
the surrounding water. These two factors act to dehydrate the
diver. Dehydration of the body and lung tissue and heat loss are
primary Decompression Sickness (DCS) Factors.

In rebreathers, the scrubbing of CO2 from the
breathing gas is an exothermic chemical reaction (it produces
heat). The reaction by-product is water vapor. The result of this
scrubbing is warm, moist breathing gas. With rebreathers, the
exhaled breath is re-circulated, which means that the moisture
level is maintained. The loop gas is typically at 100% humidity
and is much warmer than the surrounding water. This means
that the diver stays warmer and hydrated which reduces the
primary DCS factors.

Q: Rebreathers Look Technical, Are They More Difficult
to Dive?

A: The short answer is no. There is a learning curve. There is
more to keep track of than with open circuit, but after a little
while it's like driving a car, the needed skills become
automatic. The actual breathing is generally much easier than
with open circuit SCUBA.

It is important to understand, though, that rebreathers
are not for lazy or complacent individuals.

The breathing gas in a rebreather is dynamic. It can vary from
being life supporting to hypoxic (too little O2) or to
hyperoxic (too much O2). There are also concerns with
hypercapnia (too much CO2). It is not our intent to
scare people away from rebreathers, only to emphasize the
importance of training, practice and respect for the
rebreather.

Q: Do Rebreathers Require a Lot of
Maintenance?

A: Any SCUBA equipment should undergo a pre-dive check before
each use. Unlike open circuit, a rebreather requires a little
more than a couple of puffs on the mouthpiece to make sure
everything is working properly. Rebreathers require some
preparation, pre-dive checks and calibration, but these can
typically be done in 20 to 30 minutes. Post-dive cleaning can
routinely be done in about 15 minutes. An advantage is that the
pre-dive work can be done days in advance of the dive. Another
plus is that once the rig is set, you can dive the entire weekend
without touching it again (except for draining condensation, and
provided the scrubber has not been consumed).

Q: Are Rebreathers Expensive?

A: Rebreathers can cost from $2,000 for a SCR to $70,000 for
some military CCR's. An electronically controlled CCR can be
purchased for approximately $8,000, and manual-addition CCR's
such as the StingRay for approximately $6000, but reserve $1000 -
$1500 for the training. Obviously, if you only make 3 dives
annually while on family vacation, it is difficult to justify
such an expense. If, however, diving is a serious hobby or a
business, a rebreather could prove quite cost-effective. Dives
using air as a diluent are about the same cost per dive hour as
open circuit. If you are using helium-based gases, the cost
advantages of a CCR add up very quickly.

Q: Why use the KISS approach to CCR's?

A: First, the KISS approach eliminates many of the
failure-prone electronics of a CCR (after all, electronics and
water don't mix very well), while also simplifying the overall
operation of the unit.

Second, the motto of the CCR diver is "Complacency Kills".

Dr. Richard L. Pyle said,

"I return again to "Richard's Reliable Rebreather Paradox": that a more reliable rebreather can be more dangerous, because reliability encourages complacency, and as we all know, complacency kills rebreather divers more than anything else.

By this metric, manual rebreathers are the safest, because the diver has no choice but to always be cognizant of the PO2. A KISS diver is, almost by definition, an excellent rebreather diver, and one who is almost immune to the complacency killer."

By requiring that the diver monitor his
PO2, the KISS-type Manual CCR helps the diver avoid
the complacency which can be engendered by the automatic controls
of an electronic CCR. In addition, the KISS approach reduces task
loading by lengthening the interval between O2
additions from once every few seconds (for a completely manual
CCR) to once every five to ten minutes (for a properly adjusted
KISS-type CCR).

In Conclusion:

Open circuit SCUBA has reached a pinnacle of design and is now
at or near the point of diminishing returns. Rebreathers are here
to stay, and as the technology continues to advance there will be
more and more of these systems showing up on dive boats and at
dive destinations. If you have a chance to try one, do it, but be
warned that you will never look at your open circuit SCUBA gear the
same way.